This application is based upon and claims the benefit of priority from the prior Japanese Patent Applications No.P2001-250263, filed Aug. 21, 2001, and No.P2002-188506, filed Jun. 27, 2002; the entire contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to a phase difference calculation method, a phase difference calculation device, and a phase difference calculation system using an optical fiber ring interference sensor capable of detecting a phase difference between propagation lights propagated, in clockwise and counter clockwise directions, through an optical fiber loop in the optical fiber ring interference sensor.
2. Description of the Related Art
Various optical fiber ring interference sensors using optical fibers have been proposed. For example, there is a Japanese unexamined patent application, regarding an optical fiber ring interference sensor. whose application number JP-A-11/70803 filed by the inventors of the present invention.
The basic principle of this type of the optical fiber ring interference sensor is as follows.
First, as shown in FIG.8, a light emission element 11 (as a light source), a photo detector (as a light reception element) 15, and both open ends of an optical fiber loop 13 are connected to an optical splitting coupler 12. In this connection stale, the photo splitting coupler 12 splits the light emitted from the light emission element 11 as the light source. Then, the split lights are inputted to both the open ends of the optical fiber loop 13. These inputted lights are propagated in the light path B of clockwise direction (CD) and the light path A of counter clockwise direction (CCD) through the optical fiber loop 13.
The propagation lights through the light paths A and B in clockwise direction and counter clockwise direction are inputted to the optical splitting coupler 12 to couple them. The photo detector 15 inputs an interference light as the coupled light. The photo detector 15 thereby outputs a detection signal indicating an intensity change of the interference light.
That is, when a vibration is applied to a part in the optical fiber loop 13, the photo detector 15 detects a change of the difference, as a change of the interference light, between distances through which the propagation lights travel through the light path B of clockwise direction and through the light path A of counter clockwise direction.
FIG. 9 is a diagram showing a configuration of an optical fiber ring interference sensor in which the photo emission element 11 or a light input terminal, the photo detector 15 or a light output terminal, and the photo slitter-coupler 12 are connected through a pair of connection optical fibers 17. In this configuration of the optical sensor ring interference sensor, the photo detector 15 detects a vibration applied to a part in the optical fiber loop 13
FIG. 10 is a diagram showing an example of a configuration of the optical fiber ring interference sensor using a single connection optical fiber.
In each optical fiber ring interference sensor described above, the light inputted through one port of the optical splitting coupler 12 is split into a transparent light and a coupled light. Those lights are then outputted through other two ports of the optical splitting coupler 12, those two ports are located at the opposite side of the coupler 12. At this time, those lights, the transparent light and the coupled light, have a phase difference of xcfx80/2.
Accordingly, in the system configuration of each optical fiber ring interference sensor shown in FIG. 8 and FIG. 9, the phase difference xcex8 of the propagation light through the light path A and the light oath B becomes xcfx80. On the contrary, in the system configuration of the optical fiber ring interference sensor shown in FIG. 10, the phase difference xcex8 of the propagation lights in both the paths A and B becomes zero.
In this situation, when an external disturbance such as a vibration is applied to a part of the optical fiber loop 13, the phase difference between the propagation lights occurs. As a result, the photo detector 15 detects an intensity of propagation lights corresponding to the phase difference of the propagation lights changed.
The intensity Ppd of the interference between the propagation lights detected by the photo detector 15 can be expressed by the following equation (1).
Ppd=Pi+(Pc(1+cos xcex8))/2xe2x80x83xe2x80x83(1),
where, Pc is an interference component detected by the photo detector 15, and Pi is non-interference component.
Therefore, when the phase difference xcex8 is changed, the change value |dPpd/d xcex8| of the intensity Ppd of the interference becomes the minimum value when xcex8=nxcfx80(n is an integer), and the maximum value when xcex8=(n+1/2) xcfx80.
As described above, the related art has a following drawback where the sensitivity of the optical fiber ring interference sensor is extremely low because the phase difference xcex8 becomes xcfx80 under the static state in the systems of the optical fiber ring interference sensors shown in FIG. 8 and FIG. 9 and becomes zero under the system of the optical fiber ring interference sensor shown In FIG. 10, and the amount of the change of the intensity Ppd of the interference between the propagation lights detected by the photo detector is very low (specifically, becomes the minimum value at xcex8=0, xcfx80, 2xcfx80, 3xcfx80, . . . (n is an integer, for example, n=0, 1, 2, 3, . . . )) even if a vibration is applied to the optical fiber loop.
Further, following methods (a), (b), and (c) to improve the sensitivity of the sensor have been known in the related art.
(a) Method of detecting an interference signal at a timing in synchronization with a modulation signal so that a phase difference xcex8 becomes (n+1/2)xcfx80 using a lockin-amplifier (not shown) under a configuration in which a phase modulator (not shown) is inserted in an optical fiber loop;
(b) Method performed under the situation so that the phase difference between the propagation lights in paths A and B becomes (n+1/2)xcfx80; and
(c) Method to obtain a bias of a non-reciprocal phase of 2/3 xcfx80 using a 3xc3x973 optical splitting coupler.
As have clearly understood, in the various systems of the optical fiber ring interference sensors, the phase difference xcex8 between the propagation lights in the light paths A and B in the optical fiber loop detected by the photo detector 15 affects the sensitivity of the sensor.
Hereinafter, it will be considered regarding the phase difference 74 .
In order to obtain the phase differene xcex8 from the intensity Ppd of the interference between the propagation lights detected by the photo detector 15, it is sufficient to calculate the phase difference xcex8 based on the equation (1).
xcex8=cosxe2x88x921[2(Ppdxe2x88x92Pi)/Pcxe2x88x921].xe2x80x83xe2x80x83(2).
However, an inverse COS function (see the equation (2)) has innumerable solutions because it is a periodic function of 2 xcfx80.
By the way, because continuously changed, the phase difference xcex8 can be estimated based on the waveform of COSxe2x88x921 function. However, those methods involve a drawback that it is difficult to distinguish whether the current phase difference is shifted by xcfx80xe2x88x92xcex4 (corresponding to the change indicated by the dotted line xe2x80x9cbxe2x80x9d in FIG. 7) from the stationary state xcex8=xcfx80 or by xcfx80+xcex4 (corresponding to the change indicated by the line xe2x80x9caxe2x80x9d in FIG. 7) from the stationary state xcex8=xcfx80 only by using the light intensity obtained from the photo detectors,
Accordingly, an object of the present invention is, with due consideration to the drawbacks of the conventional technique, to provide a phase difference calculation method, a phase difference calculation device, and a phase difference calculation system using an optical fiber ring interference sensor capable or calculating a phase difference xcex8 between propagation lights outputted from the optical fiber ring interference sensor.
According to an embodiment, a phase difference calculation method using an optical fiber ring interference sensor. This optical fiber ring interference sensor has a light source, a plurality of photo detectors, an optical fiber loop, and a Nxc3x97N optical splitting coupler connected to the light source, the photo detectors, and open ends of the optical fiber loop. In the sensor, the Nxc3x97N optical splitting coupler splits the light emitted by the light source, the optical fiber loop inputs split lights though both open ends of the optical fiber loop, the lights are propagated through the optical fiber loop in clockwise direction and counter clockwise direction, the Nxc3x97N optical splitting coupler couples the propagation lights in clockwise direction and counter clockwise direction, and the photo detectors detect the coupled propagation light, and then outputs signals indicating a variation of a light intensity of interference between the propagation lights having a phase difference in clockwise direction and counter clockwise direction.
In order to detect a physical variation caused in the optical fiber ring interference sensor, the phase difference calculation method according to an embodiment performs measuring the light intensities of interference of a different non-reciprocal phase bias not less than two times detected from a plurality of the photo detectors, and calculating a phase difference based on the light intensities when the light intensity detected from each photo detector is within a predetermined section corresponding to each photo detector.
In addition, a phase difference calculation device according to another embodiment uses an optical fiber ring interference sensor. This optical fiber ring interference sensor has a light source, a plurality of photo detectors, an optical fiber loop, and a Nxc3x97N optical splitting coupler connected to the light source, the photo detectors, and open ends of the optical fiber loop. In the optical fiber ring interference sensor, the Nxc3x97N optical splitting coupler splits the light emitted by and supplied from the light source, the optical fiber loop inputs the split light through both open ends of the optical fiber loop, the light is propagated through the optical fiber loop in clockwise direction and counter clockwise direction, the Nxc3x97N optical splitting coupler couples propagation lights in clockwise direction and counter clockwise direction, and a plurality of the photo detectors detect the coupled propagation light, outputs signals indicating a variation of a light intensity of interference between the propagation lights having a phase difference in clockwise direction and counter clockwise direction.
In order to detect a physical variation caused in the optical fiber ring interference sensor, the phase difference calculation device according to the embodiment, connected to a plurality of the photo detectors in the optical fiber ring interference sensor, calculates a phase difference of interference between propagation lights based on light intensities obtained by a plurality of the photo detectors. That is, the phase difference calculation device measures the light intensities of interference of a different non-reciprocal phase bias not less than two times detected from a plurality of the photo detectors, and calculates a phase difference based on the light intensities when the light intensity detected from each photo detector is within a predetermined section corresponding to each photo detector.
Still moreover, a phase difference calculation system according to another embodiment has an optical fiber ring interference sensor, and a phase difference calculation device. In the system, the optical fiber ring interference sensor includes a light source, a plurality of photo detectors, an optical fiber loop, and a Nxc3x97N optical splitting coupler connected to the light source, the photo detectors, and open ends of the optical fiber loop. The phase difference calculation device is connected to a plurality of the photo detectors and calculates a phase difference between propagation lights based on a light intensity obtained by a plurality of the photo detectors. In order to detect a physical variation caused in the optical fiber ring interference sensor in the system, the Nxc3x97N optical splitting coupler splits the light emitted by and supplied form the light source, the optical fiber loop inputs split lights through both the open ends of the optical fiber loop, the lights are propagated through the optical fiber loop in clockwise direction and counter clockwise direction. The Nxc3x97N optical splitting coupler couples the propagation lights in clockwise direction and counter clockwise direction. A plurality of the photo detectors detect the coupled propagation light, and then outputs signals indicating a variation of the light intensity of interference between the propagation lights having a phase difference in clockwise direction and counter clockwise direction. The phase difference calculation device in the system measures the light intensities of interference of a different non-reciprocal phase bias not less than two times detected from a plurality of the photo detectors, and calculates a phase difference based on the light intensities when the light intensity detected from each photo detector is within a predetermined section corresponding to each photo detector.
In particular, in the phase difference calculation method, the phase difference calculation device, and the phase difference calculation system according to embodiments, the phase difference is calculated based on a first light intensity detected by a first photo detector in the plurality of photo detectors when the first light intensity is within a section corresponding to the first photo detector. Only when the first light intensity is out of the section corresponding to the first photo detector, the phase difference is calculated based on a second light intensity detected by a second photo detector adjacent to the first photo detector when the second light intensity is within the section corresponding to the second photo detector. The phase difference is further calculated sequentially based on the light intensity obtained from other photo detectors in a plurality of the photo detectors when the second light intensity is out of the section corresponding to the second photo detector.
Still furthermore, in the method, the device, and the system described above, each predetermined section is determined in advance, where the relationship between the phase difference of the propagation lights in clockwise direction and counter clockwise direction and the light intensity becomes linear.
Moreover, in the method, the device, and the system described above, each section is determined in advance, where the relationship between the phase difference of the propagation lights in clockwise direction and counter clockwise direction and the light intensity obtained from each corresponding photo detector becomes linear.
In the method, the device, and the system described above, the phase difference is calculated by using a 3xc3x973 optical splitting coupler as the Nxc3x97N optical splitting coupler and using three photo detectors as the plurality of the photo detectors.
In the method, the device, and the system described above, the phase difference is calculated by using a 3xc3x973 optical splitting coupler as the Nxc3x97N optical splitting coupler and using two photo detectors as the plurality of the photo detectors.
The above and other features and advantages of this invention and the manner of realizing them will become more apparent, and the invention itself will best be understood, from a study of the following description and attached claims, with reference had to the attached drawings showing some preferable embodiments of the invention.